Converts mash to use Shell

ash/mus/shell_delegate_mus.cc

Index: ash/mus/shell_delegate_mus.cc
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index 41b69b7b9729a556184736fc9570b35014ad4033..def94acdda616b6c1bbf8921693ffa18ce8d5bf8 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -84,10 +84,11 @@ class SessionStateDelegateStub : public SessionStateDelegate {
 
 }  // namespace
 
-ShellDelegateMus::ShellDelegateMus(service_manager::Connector* connector)
-    : connector_(connector) {
-  // |connector_| may be null in tests.
-}
+ShellDelegateMus::ShellDelegateMus(
+    service_manager::Connector* connector,
+    std::unique_ptr<SystemTrayDelegate> system_tray_delegate)
+    : connector_(connector),
+      system_tray_delegate_(std::move(system_tray_delegate)) {}
 
 ShellDelegateMus::~ShellDelegateMus() {}
 
@@ -146,6 +147,8 @@ ShelfDelegate* ShellDelegateMus::CreateShelfDelegate(ShelfModel* model) {
 }
 
 SystemTrayDelegate* ShellDelegateMus::CreateSystemTrayDelegate() {
+  if (system_tray_delegate_)
+    return system_tray_delegate_.release();
   return new SystemTrayDelegateMus();
 }